A conventional hard-disk drive typically comprises a rotatable disk on which digital information can be stored, say by means of magnetic or optical recording techniques, such that information can be read, written and erased as desired. A typical magnetic hard-disk comprises a rigid platter made from a metal or ceramic and on which a magnetic coating is provided so that information can be stored of the disk by altering the magnetic properties of the coating.
The available storage area of the disk is organised physically into a number of concentric tracks between a central portion of the disk and an outer portion of the disk. The available storage area of each track is also subdivided into a number of sectors of a fixed number of bytes and which are generally wedge or pie-shaped regions. By organising the available storage area of the disk into concentric tracks and sectors, information can be written to specific areas of the disk and stored information can be located and retrieved.
In order to read or write data to a particular location on the disk, the disk-drive typically includes a read and a write head. Traditionally induction read/write heads comprising a coil of conductive material wound over a magnetic core were used with magnetic media. Information could be written on the disk by passing an electric current through the coil to generate a magnetic field which in turn alters the magnetic properties of an area of the disk. Information could be read from the disk by passing the head over the disk such that the magnetic material of an area of the disk causes an electric current to be induced in the coil that forms the read/write head.
Following on from that magneto-resistive and particularly Giant Magneto-Resistive (GMR) heads have been employed. These are micro structures which react with a detectable change in electrical resistance to magnetic polarity or spin resonance of a region of the magnetic storage medium. They are preferable to the induction heads described above since they have a greater sensitivity to magnetic flux changes therefore allowing greater spatial resolution allowing more densely packed information storage.
A more recently proposed alternative is a Hall effect detector which is described in U.S. Pat. No. 6,353,317.
The read/write head is typically attached to one end of a movable arm. The other end of the arm is supported on a pivot or head gimbal assembly (HGA) and is connected to some form of drive means so that the arm can be selectively rotated. As the arm rotates, the read/write head moves over the surface of the disk in a shallow arc from a central region of the disk to an outer region of the disk and can thus be selectively located over any of the tracks of the disk. The drive means must be able to move the arm at high speed as information is read or written and typically comprises some form of “voice-coil” mechanism, similar to a typical drive mechanism of a loud-speaker.
In use, the disk is rotated at high speed, usually at several thousand revolutions per minute (r.p.m.), and the read/write head effectively “flies” on a cushion of air generated by the spinning disk. The read/write head is thus maintained in very close proximity to the surface of the disk without actually touching it. Indeed significant damage can be caused to the surface of the platter if the head accidentally touches it during operation. This is known as a head crash and is most likely to occur when the drive is subject to sudden physical shock. This is a potential fragility of conventional hard disk drives.
Information is either written to or read from a disk as a result of instructions generated by a computer program as the program is processed. For example, a program may generate a request for information which is then sent, typically, to a disk controller and the physical position of the data is identified using a File Access Table (FAT), the New Technology File System (NTFS), a Virtual Table Of Contents (VTOC) or other well known means. The signal requires processing and amplifying, with a number of coding protocols being used to obtain the optimum spacing and signal strength. The disk controller then causes the arm on which the read/write head is mounted to be rotated so that the head overlies the appropriate track on the disk which contains the sector in which the desired data is stored. The data cannot be read however until the correct portion of the track is brought into alignment with the read head by the spinning disk. It is therefore necessary for the disk to spin at high speed to enable the desired circumferential portion of a track on the disk to be accessed at relatively high speed, i.e. to minimise the ‘arrival time’ of the data which will on average be half the time it takes for the disk to complete one revolution.
It is well known to increase the available storage capacity of a hard drive by providing a number of disks which are mounted to and rotatable about a single central shaft. The storage capacity of the drive can also be further increased (effectively doubled) by storing information on both faces of a single disk, i.e. by providing a magnetic coating and a movable read/write head on each side of a disk. For example early mainframe drives had between one and eight disks and therefore between two and sixteen surfaces and the protocols used reflected this limit. Information stored on other types of information storage media, such as on removable floppy disks and optical information stored on removable or fixed optical disks (compact disks, DVDs and so on) is generally accessed in a similar manner. An optical disk, for example, allows information to be read (or written) by optical means, such as by a laser arrangement, however the general principles of organising information into a spiral track or a number of concentric tracks on a spinning disk and providing means to selectively position a read/write head over a desired track portion are usually followed.
As well as the above described types of information storage system, proposals have also been made to construct disk-drives that do not employ a movable arm to position a read/write head over a particular track on a disk. Instead, the disk drive is provided with a head assembly that includes one read/write head per track of an information storage disk. The head assembly generally comprises a disk-shaped element with a number of read/write heads deposited on one surface of the disk-shaped element. The heads themselves comprise generally needle-shaped elements which extend from the surface of the head assembly and which, in use, make contact with the surface of a media disk. This form of disk-drive therefore does not require a movable arm since a read/write head can be positioned over each of the available information storage tracks at all times. In this way, the inbuilt latency of more conventional disk-drives caused by the activation and movement of a head/arm assembly is removed thus enabling improved information access speeds.